Summary. The effects of decreasing oestrogen secretion upon the rate of oocyte transport achieved by the administration of 4-hydroxy-4-androstene-3,17-dione were investigated in cyclic rats. Control animals examined during late pro-oestrus showed that the majority of oocytes had entered the uterus. In contrast, when inhibitor was administered from the afternoon of metoestrus or from late dioestrus through prooestrus, oviducal retention of oocytes was observed. When treatment was delayed until the morning of pro-oestrus, only uterine retention of oocytes was observed. The inhibitor decreased oestradiol concentrations in ovarian vein, while systemic testosterone values were increased. Treatment with exogenous oestradiol counteracted the effect of the inhibitor on ovum transport. The elevation of systemic testosterone concentrations by means of subdermal implants of testosterone failed to alter the normal pattern of ovum transport. These results demonstrate that normal oestrogen secretion during late metoestrus and dioestrus is required for the movement of oocytes from the oviduct to the uterus, whereas the preovulatory oestrogen surge is needed for the expulsion of ova from the uterus.